The invention relates to an improved electrical switching system and particularly to an improved load break switch.
Electric loads drawing high alternating current from a source of high voltage are commonly connected to the source through load break switches, also termed interrupter switches. These switches are specially designed to interrupt the high load and magnetizing currents when the load is interrupted. Load break switches commonly include a main, blade like, moveable contact member adapted to rapidly close and make contact with a main stationary contact member and to rapidly open so as to break this contact. Load interruption at high voltage and high currents is subject to substantial and potentially dangerous and destructive arcing, including sustained arcing from phase to phase. One common arrangement for preventing these deleterious effects is to add auxiliary contact members to the switch. The auxiliary members briefly continue to make electrical connection across the switch while the main contact members commence opening. Thus, the main contact members do not open under power and arcing between the main contact members is prevented.
The auxiliary members generally include a movable contact member, e.g. a flipper blade, pivotally coupled to the main moveable contact member, and an auxiliary stationary contact member that is electrically connected to the main stationary contact member. When the switch is opened the main, contact members break contact while the auxiliary contact members briefly continue to make contact. Thereafter the auxiliary contacts rapidly break contact. Disengagement of the auxiliary moveable contact member, e.g. the flipper blade, from the auxiliary stationary contact member mechanically draws an arc in an arc chute. The hot arc is blown away from the rapidly moving flipper blade and cooled by contact with the large area of the chute sides and the auxiliary moveable member opens to produce a sufficient air gap to isolate the terminals of the switch.
The auxiliary stationary member frequently comprises a pair of contact members that are spring biased to abut each other. When the breaker switch is closed, the flipper blade is moved from its open position with sufficient force to stab through the abutting pair of contact members. When the switch is opened the auxiliary moveable member, e.g. the flipper blade, is briefly restrained by this pair of contact members while the main contact members break contact. The flipper blade thereafter breaks through the pair of contact members and moves to its open position.
Reliable stabbing action upon switch closure is thus required to assure that the main contact members do not open under power when the switch is opened. Many designs also rely on proper stabbing action to assure that the auxiliary moveable contact member, e.g. flipper blade, opens at a sufficiently high velocity. This rapidly extinguishes the arc when the switch is opened and thus reduces arc chute erosion. The high velocity opening results from a spring that is precharged during the brief interval when the blade is restrained. In many designs proper stabbing action is required to assure that the spring is properly precharged.
The auxiliary stationary means, therefore, have a carefully designed contact contour to insure proper stabbing. The lead in of the stationary contact members is designed to assure that the moveable member, e.g. flipper blade, properly stabs into the contact members whenever the switch is closed.
However, when the auxiliary contact members separate upon switch opening, an arc is generated in the area of the lead in contour of the stationary contact members. The severity of the arc is related to the amplitude of load current. It is particularly severe at maximum rated current. This arcing causes some erosion, e.g. pitting, of the auxiliary stationary contact members. The resulting contact erosion accumulates with repetitive use, i.e. make-break cycling, of the switch. With repeated cycling this erosion could prevent proper interfacing, e.g. stabbing, between the moveable and stationary auxiliary contact members.